The present invention relates to a plastic case formed by mating upper and lower half cases together and then welding (fusingly bonding) the two half cases together. More particularly, the invention relates to a type of plastic case which is relatively small and accordingly has limited welding spaces available.
In recent years, there have been manufactured a wide variety of products formed from plastic resins. One type of such product is a box-like case containing desired components therein. For example, a plastic case containing a recording medium such as a disk or a tape is used to form a box-like body as in a magnetic disk cartridge, a magneto-optical disk cartridge, and a magnetic tape cassette such as a video tape cassette or an audio tape cassette. This plastic case, although relatively small, is required to have high molding accuracy. The case is formed by mating upper and lower half cases together.
With respect to magnetic tape cassettes, the manner of connecting upper and lower half cases together is disclosed, for example, in Japanese Laid-Open (Kokai) Patent Applications Nos. 37913/72 and 98604/79 and Japanese Laid-Open Utility Model Applications Nos. 102085/82 and 871/89.
In the construction disclosed in the above Japanese Laid-Open Patent Application No. 37913/72, projections are formed on the connection end surface of one of the upper and lower half cases, and these projections are melted by an ultrasonic welding device so as to effect bonding. In the constructions disclosed in the above Japanese Laid-Open Patent Application No. 98604/79 and Japanese Laid-Open Utility Model Application No. 102085/82, projections are formed on the connecting portion of one of the upper and lower cassette halves, and recesses for fitting respectively on the projection are formed in the connection portion of the other. The projections are fitted in respective ones of the recesses to position the upper and lower half cases relative to each other, and then they are welded together with ultrasonic welding device. In the construction disclosed in the above Japanese Laid-Open Utility Model Application No. 871/89, concave and convex fitting portions are welded together, and then are fastened together by screws.
With respect to the plastic cases of this type used in a magnetic tape cassette, from the viewpoint of its productivity, the above publications propose to fix the upper and lower half cases together with ultrasonic welding device. For this purpose, as shown, for example, in FIG. 1, a projection 2a is formed on an end surface of a side wall of an upper half case 2, this upper case is mated with a flat distal face of a lower half case 3, and then the projection 2a is melted. However, the molten resin tends to partially run out from the mating portion thereof to the outside. As shown in FIG. 2, in order to avoid such run-out, a recess 27 is formed in the upper half case 2 while a projection 28 smaller in diameter than the opening of the recess 27 is formed on the lower half case 3, the projection 28 is fitted in the recess 27, and the contact portion between a distal face 29 of the projection 28 and the bottom surface of the recess 27 is welded. However, with this construction, it has been difficult to increase the welding area in an attempt to obtain a connecting force above a certain level. Namely, the welding area depends on the size of the distal face 29, but increasing the distal face 29 imposes a limitation on the internal space of the case. Because little excess internal space is available, it has been difficult to obtain a desired connecting force.
As a result, in the conventional magnetic tape cassettes, more expensive screw-fastened constructions have been extensively used to ensure sufficient strength as not to invite a malfunction over long periods of time.
As is well known, the above ultrasonic welding device applies ultrasonic vibrations of a frequency of about 20 KHz to about 40 KHz to the bonding surfaces while these surfaces are pressed together, whereby the synthetic resin is melted and welded by the strong frictional heat produced by the ultrasonic energy.
With respect to plastic cases of this type used in magnetic tape cassette, from the viewpoint of its productivity, it has been proposed, as discussed in the above publications, to fix the upper and lower half cases together with ultrasonic welding device, and in the case of the ultrasonic welding, constructions such as shown in FIGS. 3 and 4 have been adopted. In the case of FIG. 3, a projection 2a is formed on an distal face of a side wall of an upper half case 2, and this upper half case 2 is mated with a flat distal face of a lower half case 3, and then the projection 2a is melted. In the case of FIG. 4, a recess 27 is formed in an upper half case 2 whereas a projection 28 smaller in diameter than the opening of the recess 27 is formed on a lower half case 3. The projection 28 is fitted in the recess 27, and the region of contact between a distal face 29 of the projection 28 and the bottom surface of the recess 27 is welded.
In both of these constructions, fine particles 70 of the plastics material are produced substantially simultaneously with the welding of the bonding portions. This particles are produced by the abrasion of the bonding portions rubbing against each other as a result of the vibration of the bonding portions. The particles tend to be produced for a very short time period prior to the completion of the welding. If dust such as the fine particles 70 are retained within the case, and if an associated part contained in this case is, for example, a magnetic disk or a magnetic tape, the fine particles will adhere onto the recording surface of such media, resulting in a degradation of the recording and reproducing characteristics, and causing undesired phenomena such as drop-out in its output level.